On the Nature of ``Yield-Value''

The problem of the flow of materials at stresses far below their normal yield-values is discussed, and it is pointed out that the sharpness with which yield-values can be measured depends on the grouping of the relaxation times for the different strains set up within the material, an uneven distribution making for a sharper definition. Any sharp and drastic change in the relaxation time of the system as a whole may justifiably be said to constitute a yield-value, the question as to which of these points is actually taken as the yield-value depending on the conditions of the experiment. These considerations are reinforced by a discussion of the results (to date) of certain experiments on flour doughs, and it is claimed that flour dough is a peculiarly suitable material for such investigations. A new rapid method for studying flow in flour doughs, recently described in the literature, is discussed. The dangers of classifying materials in hard-and-fast rheological divisions is emphasized, while it is pointed out that for practical purposes, and given adequate safeguards, such classifications may be extremely useful.